In the automotive industry, rubber-coated metal parts are in widespread use. Such parts may be found in shock absorber bushings, engine mounts, or other applications where it is necessary or desirable to bond a natural or synthetic rubber to a metal substrate, often a tubular piece of metal.
By reason of the poor bonding which is obtained with straight rubber-to-metal bonds, adhesives are used which are first applied to the metal part, and thereafter forms a bond to the rubber part. Such adhesives may become cured in the same vulcanizing step in which the rubber is vulcanized and are known as post vulcanizing or PV bonding. However, pre-vulcanizing bonding materials may also be employed.
One method of applying such coating material to the parts in question has been that of conventional spray coating, while rotating the part, to form a coating of the bonding material on the outer surface. The spray coated part may the be heated to drive off the solvent. This process is inefficient in that only about 30 percent of the total adhesive sprayed is deposited on the part, and the remaining 70 percent is lost or can be recovered only with great difficulty. Dip coating also is found to have drawbacks and is inefficient in that the entire part is coated, with the coating running into the center openings or the like where it is not desired. Further dip coating may cause excessive buildup of material on the edges of the parts when the part is withdrawn from the solution, and some excess material can contribute to bond failure between the rubber and the metal at a later time.
In the patent of Robertson et al., U.S. Pat. No. 4,296,708 issued Oct. 27, 1981 and assigned to the same assignee as this invention, there is described and claimed a roll coating apparatus which has been employed to coat hollow metal sleeves or other metal parts. However, while the apparatus as disclosed and claimed in the Robertson et al patent has been successfully operated, the apparatus does not lends itself to high volume production rates, and further, the coating apparatus employed required extensive cleanup at the end of each production run and required monitoring to prevent undesired buildup of the coating material. There accordingly exists a need for a more efficient method and apparatus of applying rubber-to-metal bonding materials to rubber parts, such as hollow metal sleeves and the like.